Issue #3578375: Performance: Add static caching to FlattenedSiteSettingsLoader::loadAll()

Problem/Motivation

The FlattenedSiteSettingsLoader::loadAll() method is called via site_settings_preprocess() for every Twig template rendered on a page. Since the loader plugin is instantiated fresh each time via getActiveLoaderPlugin(), the instance property $this->settings is always NULL, causing a cache backend lookup on every single call.

On a typical node edit page with ~4,000 templates, this results in ~4,000 unnecessary cache lookups per request.

Performance Impact

Metric	Without Fix	With Fix
loadAll() cache lookups	4,085	1
MemcachedDriver::getMulti calls	4,933	~850
Wall time for site_settings	250ms	<1ms
Total page load	3.1s	~2.6s

Solution

Add a static cache ($staticSettingsCache) that persists across plugin instances within a single request:

1. Check static cache before backend cache lookup
2. Store results in static cache after loading
3. Clear/update static cache in clearCache() and rebuildCache()

Changes

• Added protected static $staticSettingsCache = [] property
• Modified loadAll() to check static cache first
• Modified rebuildCache() to update static cache
• Modified clearCache() to clear static cache

Related Issues

• Similar fix applied to Metatag module: https://www.drupal.org/project/metatag/issues/3577367